In order to assist understanding of the invention which will be explained in the following text, the basic design and method of operation of an electronic ballast which is used to drive a fluorescent lamp, and its method of operation, will first of all be explained with reference to FIGS. 1 to 3. One such ballast is described, by way of example, in EP 1 066 739 B1, in U.S. Pat. No. 5,973,943 or in U.S. Pat. No. 6,617,805 B2.
The ballast has a half-bridge with a first semiconductor switching element Q1 and a second semiconductor switching element Q2, whose load paths are connected in series between terminals K1, K2 between which a DC voltage Vb is applied. This DC voltage Vb is produced (in a manner which is not illustrated in any more detail), for example, by means of a generally known power factor correction circuit (Power Factor Controller PFC) from a mains AC voltage. This DC voltage Vb has a normal amplitude value of 400 V.
The half-bridge circuit Q1, Q2 uses this DC voltage Vb to produce a voltage V2 with a pulsed signal waveform at an output K3. The two semiconductor switching elements are driven in a pulsed manner by a drive circuit 20 via drive signals S1, S2 in order to produce this pulsed voltage V2. This drive is intended to minimize switching losses, such that the two switching elements Q1, Q2 are never switched on at the same time, and such that both switching elements are switched off for a predetermined time period at the same time during a switching process. The frequency with which the two switching elements are driven in a pulsed manner and at which the pulsed voltage V2 is produced is dependent, inter alia, on the burning state of the fluorescent lamp 10 that is supplied by the circuit and, once the lamp is burning, is, for example, 40 kHz. This frequency is adjusted by the drive circuit in a fundamentally known manner. Signal inputs via which the drive circuit receives information about the burning status of the lamp, and apparatuses for production of such signals, are not illustrated in the figures, for clarity reasons. The figures likewise do not show circuit components for supplying voltage to the drive circuit.
The fluorescent lamp 10 is connected in parallel with a resonant capacitor C1 which is part of a resonant tuned circuit. This resonant tuned circuit which, in addition to the resonant capacitor C1, has a resonant inductance L1 connected in series with the resonant capacitor C1, is connected to one output K3 of the half-bridge Q1, Q2 and is supplied by the pulsed supply voltage V2. A blocking capacitor C2 which is connected in series with the resonant tuned circuit L1, C1 is used to filter out the DC voltage component from the pulsed supply voltage V2, thus resulting in an AC voltage with an approximately square or trapezoidal signal waveform across the arrangement with the resonant tuned circuit L1, C1 and the fluorescent lamp 10. The amplitude of this AC voltage is approximately half the magnitude of the DC voltage that is applied to the half-bridge Q1, Q2.
After being started, the fluorescent lamp 10 behaves like a voltage-dependent resistance. A voltage which is dropped across the lamp 10 after it has been started has a waveform which approximates to a sinusoidal curve.
Before the lamp 10 is started, the lamp electrodes 11, 12 must be preheated to an emission temperature. For this purpose, the supply voltage V2 is produced at a higher frequency than after starting, thus resulting in a voltage V10 which is less than a burning voltage on the lamp 10. After the end of the preheating phase, the drive frequency of the half-bridge circuit Q1, Q2 is reduced in order to reach a burning voltage, which is sufficient for the lamp to burn, and thus to start the lamp.
In order to preheat the lamp electrodes 11, 12, the lamp may be connected in the resonant tuned circuit in various ways. In the example shown in FIG. 1, the current in the resonant tuned circuit L1, L2 flows through the electrodes 11, 12, in order to preheat them. In the example shown in FIG. 2, auxiliary inductances Lh1, Lh2 are provided for preheating of the electrodes 11, 12, are inductively coupled to the resonant inductance L1 and are respectively connected to one of the electrodes 11, 12 in order to preheat them.
The arrangement with the resonant tuned circuit L1, C1 and the fluorescent lamp 10 can be connected, with reference to FIGS. 1 and 2, between the output K3 of the half-bridge circuit Q1, Q2 and a reference ground potential GND, or with reference to FIG. 3 between the output K3 of the half-bridge circuit Q1, Q2 and the center tap of a capacitive voltage divider C4, C5 which is connected between the input terminals K1, K2.
A snubber capacitor C3 is connected in parallel with the load path of the second semiconductor switching element Q2 of the half-bridge circuit, with the object of allowing zero voltage switching operation (ZVS) of the two semiconductor switching elements Q1, Q2.
Fluorescent lamps have a finite life. Towards the end of this life, when the lamp is worn, the emission capability of the lamp electrodes 11, 12, which emit electrons into a fluorescent gas during operation, falls. As these electrons move from the metal of the electrodes 11, 12 into the gas discharge, this normally actually results in a sufficiently large amount of heat being produced to keep the electrodes 11, 12 at the temperature that is required for emission. If these emission conditions deteriorate as a result of wear, then a greater voltage drop occurs on the electrodes, and this leads to a larger amount of heat being produced, and to poorer lamp efficiency. While relatively old lamp types were normally able to withstand locally greater power loss without damage owing to their larger dimensions, this greater power loss and the greater amount of heat that is produced resulting from it in the case of relatively new lamp types, for example in the case of lamps with a diameter of ⅝″, can in the extreme lead to the glass surrounding the lamp melting. It is therefore necessary to identify the end of the life of fluorescent lamps in good time, in order to avoid such damage.
When the end of the life of a lamp is reached, the voltage V10 across the lamp rises. One of the two electrodes 11, 12 will normally wear earlier than the other, so that the lamp voltage V10 becomes unbalanced, that is to say one of the positive or negative half-cycles has a greater amplitude than the respective other half-cycle. Based on this knowledge, it is known for the wear of a fluorescent lamp to be detected by forming the arithmetic mean value of the lamp voltage and comparing this with zero. If this arithmetic mean value differs by more than a predetermined amount from zero, thus indicating an unbalanced lamp voltage, it is assumed that the end of life has been reached.
Methods such as these, in which the arithmetic mean value of the lamp voltage is evaluated for wear detection, are described by way of example in U.S. Pat. No. 5,808,422 or EP 0 681 414 A2. These methods make use of the fact that the arithmetic mean value of the lamp voltage V10 plus half the supply voltage Vb is dropped on the blocking capacitor c2, and can thus be measured and monitored relatively easily.
The known methods have the disadvantage that their implementation requires a comparatively large number of components, which cannot be integrated.